The present invention relates to a coreless motor having a permanent magnet fixed to a casing and formed with an even number of poles, a rotor coil unit integrated with an output shaft, and a commutator.
There has been known the conventional coreless motor of the cylindrical type used in an office automation instrument, a robot, a medical instrument and so on, which requires a high performance motor featuring a high efficiency as well as a quick start/stop motion.
FIG. 10 is a structural diagram of the prior art motor. In a manner similar to other types of motors, there is included a rotor 5 and a stator composed of a magnet 4. The rotor 5 includes a rotor coil unit 7 which solely contributes to generation of output torque. The motor further comprises a rotor holder 6 which supports the coil unit, a commutator mechanism composed of commutator segments 8 for controlling rotation direction, and an output shaft 9 for rotatably supporting the rotor. The stator is constructed such that a bearing housing 2 having bearings 3 contains therein the fixed magnet 4. A casing 1 is fixed to the bearing housing 2 to cover entirely the same, while the casing 1 functions as a return yoke of the magnet 4. An electric current is fed to the rotor coil unit 7 of the rotor 5 via lead wires 12, contacts of a brush 10 and respective commutator segments 8. A pair of washers 13 are disposed on outer faces of the respective bearings 3, and a stopper ring 14 is fixed to the output shaft 9 in order to suppress an axial movement of the rotor 5 during the course of rotation. A cap 15 is fixed to an inner portion of a rear cover 11 to block an exterior dust.
The conventional coreless motor has a winding structure shown in FIGS. 11, 12 and 13. The rotor coil unit 7 of the coreless motor is connected electrically as shown in FIG. 14 to form the rotor 5 as shown in FIG. 16. As understood from FIG. 16, this coreless winding is formed such that the coil unit 7 of the rotor 5 has a radial thickness defined by two layers of the windings. However, since the radial thickness is limited to twice the diameter of the coil wire, the conventional structure has the drawback that the coil wires cannot freely be wound thick, thereby limiting the amount of copper in the coil unit.
Particularly in reducing the motor size, while an energy product of the magnet has been improved efficiently, a magnetomotive force of the coil of the rotor has not been improved efficiently. Stated otherwise, in reducing the motor size, the magnetic loading has been improved while the electric loading has not been improved. The motor output torque cannot be optimally improved unless a design balance is ensured with respect to a ratio between the magnetic loading and the electric loading. In view of this, it is necessary to optimally broaden a space gap between the magnet and the casing so as to increase the amount of copper in the rotor coil unit. In order to increase the copper amount of the coreless coil, it is necessary to increase a radial thickness of the cylindrical coil unit.
It might be advisable to form multiple stages of the cylindrical coil units. However, for example, when respective stages of the coil units are connected in parallel to each other as shown in FIG. 15, drawback that an inductive voltage coefficient Ke cannot be raised in the reduced motor. There is a problem that a series connection is needed in order to increase a value of Ke in the prior art.